I. Field of the Invention
The present invention relates in general to varistors, and more particularly to such varistors having a symmetrical structure for surface mount applications.
II. Description of Related Art
Varistors, and especially metal oxide varistors, have gained widespread acceptance as devices for providing a nonlinear resistance function. The electrical characteristics of such voltage-dependent resistors are expressed in part by the relation: EQU I=(V/C).sup.n
where V is the voltage across the varistor, I is the current flowing through the varistor, C is a constant corresponding to the voltage at a given current, and the exponent n has a numerical value greater than 1. The value of n is calculated according to the following relation: ##EQU1## where V.sub.1 and V.sub.2 are the voltages at currents I.sub.1 and I.sub.2, respectively. The desired value for C depends upon the type of application in which the varistor is to be used. It is ordinarily desirable that the value of n be as large as possible, since this exponent determines the degree to which the varistor departs from Ohmic characteristics.
Although substantial effort on the part of many investigators has led to increasing understanding of the characteristics and methods of operation of metal oxide varistors, the device is nevertheless not completely understood. For this reason, many significant improvements in varistor operation are made more or less heuristically, and the reasons for the improvement or mechanism or the accomplishment thereof are not always known with complete certainty.
It is known, however, that the electrical properties of a varistor are determined primarily by the physical dimensions of the varistor body. The energy rating of a varistor is determined by the volume of the varistor body, the voltage rating of a varistor is determined by the thickness or current path length through the varistor body, and the current capability of the varistor is determined by the area of the varistor body measured normal to the direction of current flow.
The term "surface mount varistor" is generally used to describe a varistor in which both the input and output terminals are positioned on the same major surface of the varistor body. Surface mount varistors are particularly adapted for applications in which the varistor is to be placed upon, for example, a printed circuit board. In such applications, the conductive surfaces of the input and output terminals are typically positioned directly above the conductive runners of the printed circuit board. Solder paste is positioned between the conductive surfaces of the input and output terminals and the respective conductive runners of the printed circuit board. The entire assembly is then heated, causing the solder to melt and producing an electrical contact between the varistor terminals and the printed circuit board.
In such applications, it is essential that the varistor be properly oriented with respect to the printed circuit board prior to soldering. If the surface mount varistor is improperly oriented; i.e., if the major surface of the varistor on which the input and output terminals are positioned faces away from the printed circuit board, electrical contact between the circuit board runners and both terminals of the varistor will not be made. As a result, the circuit of the assembled printed circuit board will not function as intended. The requirement of verifying the proper orientation of the surface mount varistor prior to assembly adds considerable time and expense to the assembly process.
It is an object, therefore, of the present invention to provide a varistor having a plurality of major surfaces, each of which provides both an input and output terminal thereon.
It is another object of the present invention to provide a surface mount varistor having input and output terminals symmetrically disposed on opposed major surfaces thereof.
It is a further object of the present invention to provide a surface mount varistor which is fully passivated.